The present invention relates to an electroconductive device for use in a flat panel display, a projection display, a printer, etc.
Hitherto, liquid crystal display devices have been most extensively used among flat panel display devices as space-economical man-machine interfaces. Particularly, a so-called active matrix (e.g., TFT)-type liquid crystal display device, wherein each pixel is equipped with an active element, such as a transistor, is predominantly being used since the price thereof is dropping because of improvements in production technology.
However, the nematic liquid crystals most extensively used have slow response times of almost several milliseconds, which is problematic in high-speed drawing, such as motion picture display. Further, since the birefringence of the liquid crystal varies depending on the viewing direction, the liquid crystal display is accompanied by the problem of large viewing angle-dependence.
In order to obviate the above-mentioned problems, self-luminescence-type devices capable of providing flat panels have been used in recent years, including plasma luminescence display devices, field emission devices and electroluminescence devices.
Electroluminescence devices (hereinafter, the term xe2x80x9celectroluminescencexe2x80x9d is sometimes abbreviated as xe2x80x9cELxe2x80x9d according to common usage in the field) may be classified into inorganic EL devices and organic EL devices. An inorganic EL device is an AC-driven thin-film EL device using an inorganic semiconductor, such as ZnS. Such a device is excellent in gradation characteristic and luminance, but is problematic since it requires an AC drive voltage on the order of several hundred volts.
On the other hand, for an organic EL device, T. W. Tang, et al. demonstrated in 1987 that high-luminance light emission was realized at a low DC voltage by utilizing a laminated structure of thin films of a fluorescent metal chelate complex and a diamine molecule. The device was an organic EL device including a laminate of the above-mentioned very thin layers (a charge-transporting layer and a luminescence layer) respectively formed by vacuum deposition sandwiched between an anode and a cathode.
An organic EL device is a carrier injection-type self-luminescent device that utilizes the luminescence caused at the time of recombination of electrons and holes having reached a luminescence layer. An organic EL device has a laminated structure of cathode/organic compound layers/anode, and the organic compound layers generally assume a two-layer structure including a luminescence layer and a hole-transporting layer or a three-layer structure including an electron-transporting layer, a luminescence layer and a hole-transporting layer.
In the above structure, the hole-transporting layer has a function of effectively injecting holes from the anode into the luminescence layer, and the electron-transporting layer has a function of effectively injecting electrons from the cathode into the luminescence layer. Simultaneously, the hole-transporting layer and the electron-transporting layer have functions of confining electrons and holes, respectively, to the luminescence layer (i.e., carrier-blocking functions), thereby enhancing the luminescence efficiency.
As for these charge-transporting layers, the most important property is believed to be carrier mobility. An organic compound in an amorphous state generally shows a carrier mobility on the order of 105 cm2/Vsec, and this is not regarded as a sufficient transportation performance. It is believed that the luminescence efficiency is enhanced if the mobility of the charge-transporting layer can be increased so as to inject more carriers into the luminescence layer. Moreover, a higher mobility, if achieved, can allow an increase in thickness (generally on the order of several hundred xc3x85) of a charge-transporting layer to a thickness on the order of several xcexcm, so that the productivity can be increased while preventing short circuit between the electrodes or between the layers sandwiching the charge-transporting layer. For these reasons, extensive research and development work has been conducted on various compound materials which make up the charge-transporting layer in order to achieve higher efficiencies for organic EL devices.
Under the circumstances, there is a trend to impart a liquid crystal property to an organic compound which constitutes the charge-transporting layer. The organic film used in an organic EL device is generally in an amorphous state, and the molecular alignment therein lacks regularity. In contrast thereto, some liquid crystalline organic compounds having a certain order of molecular alignment were found to exhibit a high mobility and have gained interest in the field. For example, Haarer, et al., observed a high hole mobility of 10xe2x88x921 cm2/Vsec in the mesophase of a long-chain triphenylene compound that is a representative discotic liquid crystal. (Nature, vol. 371, p. 141 (1994)). Haarer, et al., also studied the relationship between the degree of molecular alignment order and hole mobility in the columnar phase of a series of triphenylene-based discotic liquid crystals and reported that a higher degree of order provided a higher hole mobility (Add. Mater., vol. 8, p. 815 (1996)).
Further, Hanna, et al. reported that a bar-shaped liquid crystal having a phenylnaphthalene skeleton in smectic E (SmE) phase exhibited a mobility of 102 cmxe2x88x922/Vsec as a bipolar electronic conductivity for both electrons and holes (Appl. Phys. Lett., vol. 73, no. 25, p. 3733 (1998)).
Based on a self-alignment characteristic and a possibility of molecular alignment control, which is advantageous for carrier transportation of a liquid crystalline organic compound as mentioned above, it is possible to realize an excellent carrier-transporting material. Further, by enhancing the carrier-transporting performance, the applicability of the device can be extended to other electronic devices including switching devices, such as transistors.
As described above, a liquid crystalline organic compound can exhibit an excellent performance for carrier transportation and is promising for providing a charge-transporting layer of an electroconductive device.
As for actual utilization, a liquid crystalline organic compound has a mesophase (mesomorphic phase, i.e., liquid crystal phase) temperature range essentially determined by chemical structure, etc., of the compound, and all the liquid crystalline organic compounds do not show a broad mesophase temperature range. Further, there are many compounds which assume a mesophase only at high temperatures and do not show mesomorphism in a temperature range (around room temperature, e.g., 0xc2x0 C.-40xc2x0 C.) suitable for practical device use.
Accordingly, it is very important to provide a liquid crystalline organic compound that exhibits stable mesomorphism over a broad temperature range when laminated and disposed between electrodes as an actual form of use in an electroconductive device.
In view of the above-mentioned problems of the prior art and the above-mentioned subject, an object of the present invention is to provide an electroconductive device including a liquid crystalline organic compound layer exhibiting a stable mesomorphism as a charge-transporting layer.
Another object of the present invention is to provide an electroconductive device suitable for use in an EL device that is capable of exhibiting good luminescence characteristics.
According to the present invention, there is provided an electroconductive device comprising a pair of oppositely disposed electrodes and at least one organic compound layer disposed between the electrodes so as to be supplied with a voltage applied between the electrodes, said at least one organic compound layer including at least one layer of liquid crystalline organic compound assuming a supercooled liquid crystal phase in the state of being disposed between the electrodes.
In the electroconductive device of the present invention as described above, the mesomorphism of the liquid crystalline organic compound can be stabilized to allow the liquid crystalline organic compound layer to exhibit a better charge-transporting performance, which results in an improved luminescence performance when included in an EL device.